A heart attack results in the death of cardiomyocytes (CMs). Most adult mammalian CMs are post-mitotic, which effectively prevents regeneration and restoration of function following damage. Recent studies have demonstrated that CM cell division does occur in adult mammals, albeit to a highly limited extent; this implies the existence of a small subset of CMs that has the capacity to initiate cell division and achieve some measure of regeneration. Unfortunately, the extent to which this occurs in most people is too little to restore normal heart function, such that heart failure remains the leading cause of death in the Western world. Strong evidence supports the model that mononuclear, diploid CMs, a relatively rare population in the adult mammalian heart, are a regeneration-competent population. This project explores the hypothesis that the frequency of these mononuclear, diploid CMs in the adult mammalian heart and regenerative potential are two interlinked variable traits determined by multiple genetic parameters. If so, different individuals will have a greater r lesser capacity to undergo heart regeneration following injury, based on their unique genetic backgrounds. Preliminary studies in mice demonstrate a substantial variation in the number of mononuclear, diploid CMs across different inbred strains and this correlates with both frequency of DNA division and improved cardiac function following injury. An extension of this model is that association studies could identify the genetic loci responsible for this observed variation. The Hybrid Mouse Diversity Panel, a collection of inbred mouse strains designed to expedite genetic association studies of this type, will be used to identify the genes correlated with the frequency of mononuclear, diploid CMs. So far, two loci located on chromosome 3 have already risen to significance with association studies. This proposal will validate the candidate gene(s) within the existing highest priority locus for their role in maintaining the mononuclear, diploid population b genetically manipulating the expression of the gene in a mouse model and quantifying the effect of that genetic manipulation on the population. A second experiment will assess the effect the validated gene(s), and therefore the population, have on the ability of the heart to regenerate after injury, thereby definitively link the mononuclear, diploid CM population to heart regeneration.